The efficiency of, for example, a gas turbine engine is dependent upon, inter alia, the clearance between the tips of its blades and the casing and in particular compressor and turbine blades and their surrounding casing. Thus the smaller the clearances, the lower is the gas leakage across the aerofoil tips. However under certain engine conditions the aerofoil blades and their associated discs may experience thermal growth and there is an increased risk of contact with the casing.
It is an essential part of a gas turbine engine's development to accurately measure the clearance between the tips of blades and the associated casing. Prior art systems for providing such measurement include the use of a probe mounted on the casing surrounding the blades. The blade tip and probe act in effect as the two plates of a capacitor. The probe relies on a capacitance change within the clearance between the blade and casing/probe. More specifically the capacitors defined by the aerofoil blade tips and the probe are used as the frequency determining element in the tuned circuit of a high frequency oscillator. As the blade tip clearances alter so do the capacitances of the formed capacitors and hence the frequency of the oscillator. The oscillator output is then used to provide a measurement of the clearance between the blade tips and casing.
One prior art system for measuring blade tip clearances is disclosed in GB 2071852A and reference is directed thereto. An oscillator is provided remote from the blade tip and is connected to a conductor which forms part of a probe positioned close to a blade tip. The output frequency of the oscillator is a function of each of the capacitors defined by the probe, the blade tips and the inductance of an inductor which constitutes part of the oscillator.
During certain operating conditions the probe may be subject to temperature changes. Such temperature changes result in corresponding changes in the dielectric constant of the insulating material of the conductor and the capacitance correspondingly changes. It has previously been proposed to provide a guard electrode within the probe and connected to the oscillator. The guard electrode serves to ensure that any changes in the dielectric constant of the insulating material have no effect on the frequency of the oscillator.
Even relatively small changes in temperature may cause errors in the system which are many times greater than the distance related capacitance. However the length of the cable used with a guard electrode is limited because of difficulty in driving the large guard capacitance at high frequencies.
Thus if the capacitance increases to such an extent then it is not possible to use the probe further. It is therefore necessary to position the oscillator circuit close to the probe which is undesirable in high operating temperatures.